To determine the prevalence of obesity among children presenting to the emergency department (ED) with acute asthma, and to examine the relationship between body mass index (BMI) and acute asthma severity in the ED setting. We analyzed data from a multicenter prospective cohort study during 1997-1998; 44 ED in 17 US states and two Canadian provinces enrolled 672 patients, age 5-17, with acute asthma. BMI and Pulmonary Index were collected in the ED. We defined overweight and obesity using age, sex, and race-specific BMI values from national and international databases. The prevalence of obesity was significantly higher among ED patients with acute asthma as compared with children from the general population (23% vs. 9-15%; p < 0.001). Obese children with acute asthma did not differ from their non-obese counterparts, by demographic factors or chronic asthma severity (all p > 0.2). Initial Pulmonary Index was the same across underweight, intermediate, and obese groups (3.7 +/- 2.4, 3.8 +/- 2.2, 3.7 +/- 2.3; p = 0.70). Admission status also did not vary across groups (22%, 22% and 23%; p = 0.98). Stratifying the analysis by age group and sex did not change these results. The prevalence of obesity among children presenting to the ED with acute asthma was significantly higher compared with children from the general population. BMI was not associated with markers of chronic and acute asthma severity. The results of this study support a positive association between obesity and asthma, and suggest that asthma exacerbations among obese children are very similar to those experienced by other children.